1. Field of the Invention
The present invention relates to an automatic focusing apparatus for use in a semiconductor lithography system, a semiconductor inspection system and the like, and particularly to an automatic focusing apparatus for focusing an objective lens on the surface of substrates to be inspected, such as a semiconductor substrate or a glass substrate.
2. Description of the Related Art
A resolution which is equal to or less than a circuit pattern size is required in optical systems such as a lithography system which transfers a circuit pattern onto a semiconductor substrate, and a defect inspection system which inspects a pattern of a semiconductor integrated circuit. In an optical apparatus in which a light source in a range of visible light to ultraviolet light is used, in order to increase the resolution, in general, it is necessary to shorten a light source wavelength or to increase a lens numerical aperture (N/A). However, since a depth of focus of the optical system is proportional to the light source wavelength, and is inversely proportional to a square of NA, a short-wavelength/high-NA optical system has a problem that the depth of focus is small. Therefore, an automatic focusing apparatus is indispensable which keeps a distance between substrates to be inspected such as a semiconductor substrate and an objective lens to be constant, and enhancement of precision of the automatic focusing apparatus is required.
As a conventional automatic focusing apparatus, there is an oblique light incident method disclosed, for example, in Jpn. Pat. Appln. KOKAI Publication No. 6-102011. In this method, the surface of the substrate is irradiated with light from an oblique direction, a reflected light from the substrate is detected by a photosensor, and a detection output of the photosensor is converted to positional deviation information of a vertical direction to measure a vertical position. Moreover, when a measurement signal is fed back to a control circuit of a stage, a relative distance between the substrate and a main optical system can be maintained with high precision.
Moreover, as another method, there is a double pinhole method described in Jpn. Pat. Appln. KOKAI Publication No. 5-297262. In this method, the light reflected by the surface of the substrate is converged by a lens, and is also branched into two light paths by a beam splitter, a pinhole and a photosensor are disposed before a focal point of the lens in a first light path, the pinhole and photosensor are disposed after the focal point of the lens in a second light path, and an electric difference/sum of signals obtained by the two photosensors is calculated. Accordingly, the position of the substrate surface in the vertical direction can be measured.
However, in this type of automatic focusing apparatus, there is a possibility that the following problem is generated. That is, when the vertical position of the surface of the substrate is detected while moving a stage including a substrate laid on the stage in a horizontal direction, a steep change exceeding a change of an original surface shape of the substrate is caused.
With respect to this problem, the present inventors have found that a phenomenon in which a reflection spot position of reflected light shifts with respect to a fine pattern having a line width of the same degree as that of a light source wavelength, when an incident angle θ between a pattern surface and incidence light exceeds a certain value. This is caused by resonance, which occurs when a spatial period of a pattern and an incident angle and a wavelength satisfy a specific relation. The shifting of the reflection spot is regarded as movement of a focal position in a photosensor plane. Therefore, an output of a position detection circuit changes in proportion to a movement amount of a spot position, and a large error is made when the vertical position of the substrate surface is measured.
When a light beam is incident upon a fine pattern on the substrate having the same degree of line width as that of the light source wavelength in the conventional automatic focusing apparatus in this manner, the spot position of the reflected light changes, and accordingly there has been a problem that a measurement error is generated.
Therefore, there has been a demand for realization of an automatic focusing apparatus capable of suppressing the measurement error caused by the spot position change of the reflected light and capable of enhancing reliability of focusing, when the light beam is incident upon the fine pattern having the same degree of line width as that of the light source wavelength of the substrate.